Critical Speed for Gas Induction

The critical speed for gas induction is solely decided by the ability of the rotor to generate a suction higher than the sum of static head and other pressure losses. The rotor design obviously plays an important role. Even when a second impeller is employed for gas dispersion/solid suspension, its action is limited to the role assigned to it in a region substantially away from the gas-inducing device. Therefore, the presence of the second impeller should not have any effect on the critical speed for gas induction in a multiple-impeller system. This obvious fact was experimentally proved by Saravanan and Joshi (1995). Consequently, Equations 9.23 or 9.28, which yield similar predictions, can be used to predict The effect of liquid viscosity can be accounted through Equation 9.24. 

FIGURE 9.8 Flow patterns of (a) single PTD, (b) PTD-PTU, and (c) PTD-PTD multiple-impeller systems. (Reproduced from Patwardhan and Joshi, 1999 with permission from American Chemical Society. Copyright 1999 American Chemical Society.) 

FIGURE 9.9 Various geometric parameters relevant to multi-impeUer gas-inducing system as used by Saravanan and Joshi (1995). (Reproduced from Saravanan and Joshi, 1995 with permission from American Chemical Society. Copyright 1995 American Chemical Society.) 

FIGURE 9.10 Variation of gas induction rate with interimpeller clearance. (Data generated from Fig. 14B of Saravanan and Joshi 1995.) 

1 Recommended Configuration for Maximum The overall picture that 

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In a gas-inducing reactor, both gas and liquid phases are generally considered to be completely backmixed. The use of Eqs. (2.39) and (2.40) for the calculations of the critical speed for gas induction is recommended. The rate of gas induction can be expressed by a dimensionless relation NA = /(FrdJH, Ga, dT/du HJdj). The most important parameters are Fr dJH and dT/di. For a given power input per unit volume, the turbo aerator appears... 

Replacing N by N defines the flow number at the critical speed for gas induction. 

It is desirable to know a priori the minimum (or critical) impeller speed, AI at which the onset of gas induction occurs. The onset of induction will occur when the static pressure at the point of gas induction equals the static pressure in the gas space. For pipe and flattened cylindrical impellers, Joshi and Sharma (1977) proposed... 

The most extensive test which has been made of this conduction model for thermal explosion is to be found in the work of Vanp e on the explosion of CH2O + O2 mixtures. He used a calibrated thread of 10 per cent Rh-Pt alloy of 20 m diameter (jacketed by a 50-m quartz sleeve) suspended at the center of a cylindrical vessel to measure directly his reaction temperature during the induction periods preceding explosion. By Uvsing He and Ar as additives and vessels of different diameters he was able to verify the dependence of the critical explosion limits on vessel size and on thermal conductivity of the gas mixture. In addition, he was able to check the maximum predicted temperature at the center of the vessel just prior to explosion and also the value of 8c = 2 [Eq. (XIV.3.12)], the critical explosion parameter for cylindrical vessels. Finally, with a high-speed camera, he was able to show directly that the explosions in this system do start at the center, the hottest region, " and propagate to the walls. 

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